foreach (var section in document.Sections)
{
    foreach (var block in section.Blocks)
    {
        if (!(block is Table table))
        {
            continue;
        }

        foreach (var row in table.Rows)
        {
            foreach (var cell in row.Cells)
            {
                if (!cell.Properties.Padding.HasLocalValue)
                {
                    cell.Padding = table.TableCellPadding;
                }
            }
        }
    }
}